Aliphatic and cycloaliphatic anhydrides such as commercial grades of dodecenyl succinic anhydride (DDSA) and of methyl tetrahydrophthalic anhydride (MTHPA) are used as curing agents in the production of epoxy resins. In some cases these aliphatic and cycloaliphatic anhydrides are used individually and in some cases they are used in combination with each other. If DDSA is used by itself, the resultant epoxy resin has a relatively low glass transition temperature (in the order of about 80.degree. C.). On the other hand, use of MTHPA produces epoxy resins having higher glass transition temperatures (in the order of about 140.degree. C.). Therefore, mixtures of DDSA and MTHPA are used to provide epoxy resins having physical properties required for use under various actual service conditions, such as, for example, desired glass transition temperatures, suitable polymer modulus, and the like.
Heretofore, the desired mixture of DDSA and MTHPA has been produced either by purchasing the individual products and mixing them together at the polymerization site prior to conducting the polymerization or by purchasing a preformed mixture of the DDSA and MTHPA from the supplier. So far as is known, the supplier forms such preformed mixtures from DDSA and MTHPA which have been produced in separate chemical operations.
Typically, the alkenyl succinic anhydrides are formed by reacting a branched olefin such as propylene tetramer with maleic anhydride. During the reaction it is desirable to consume the majority, if not all, of the maleic anhydride so that little if any unreacted maleic anhydride will need to be recycled. One way of improving the consumption of the maleic anhydride during the reaction is to perform the reaction at an elevated temperature. In the case of DDSA production temperatures as high as 250.degree. C. are required in order to achieve sufficient reaction kinetics (rapidity of reaction). Unfortunately at such temperatures, side reactions may occur even though short reaction times are used. Such side reactions can and often do result in formation of products having unattractive dark coloration. From a commercial standpoint, it would be advantageous to provide a product possessing improved color characteristics.
A need thus exists for a method in which the DDSA can be produced rapidly and without appreciable color development during its synthesis thereby obviating the need for use of purification procedures such as distillation or decolorization.
The conventional method for producing commercial grade MTHPA involves the reaction of maleic anhydride with isoprene, piperylene, or butadiene, or a mixture containing these dienes. The resultant product of this Diels-Alder reaction is a solid at room temperature. A liquid form of this product is highly desired in the marketplace. To fulfill this requirement, it has been the practice heretofore to subject the product either to hydrogenation whereby the double bond is saturated by hydrogen, or to isomerize the product usually in the presence of a catalyst. Both such operations are not only time consuming, but add substantial capital and processing costs to the operation.
A need thus exists for a way of producing a liquid product containing MTHPA without requiring recourse to hydrogenation or isomerization.